Little is known about the development of the cellular features of the constituent neurons and the extremely complex patterns of synaptic connections in the mammalian cerebral cortex. We propose to perform a comprehensive qualitative and quantitative analysis at the light and electron microscope levels of: (1) the development of the cellular features of various neuron types in the visual cortex of the rat, and (2) the temporal and spatial characteristics of synaptogenesis in the same cortical area. The visual cortex of the rat, in which neuronal differentiation and synaptogenesis occur for the most part in the early postnatal period, is a particularly favorable region in which to attempt such studies, which are in any case an essential prerequisite for studies designed to investigate the effects on the development of the visual cortex of various experimental, environmental and genetic factors.